1. Field of the Invention
The present invention relates to a silicon nitride sintered body with an improved strength near room temperature.
2. Description of the Related Art
Silicon nitride sintered bodies have recently attracted attention for their high strength and chemical stability, and their use in mechanical parts and the like has been expanded. With such increase in use, the effects of impurities contained in silicon nitride sintered bodies on their properties have frequently studied.
As one of results of curent studies, the proceeding of 1993 Annual Meeting of JAPAN Ceramic Society "The Effect of Halogen Impurities in Raw Powder on Mechanical Properties of Si.sub.3 N.sub.4 Sintered Bodies" (M. Asayama, A. Tsuge, S. Kanzaki, K. Watari, K. Isozaki, H. Hiroturu) describes that halogen elements (fluorine, chlorine) contained in silicon nitride sintered bodies showed lower high-temperature strength (flexural strength at 1250.degree. C.).
The article "High-Temperature Strength of Fluorine-Doped Silicon Nitride" in Journal of the American Ceramic Society, Vol. 77, No. 1 (1994), pp. 275-277 describes that fluorine contained in silicon nitride sintered bodies causes the degradation of creep resistance at high temperature (1400.degree. C.).
Both of these studies suggest that the reduction of contents of halogen elements (fluorine, chlorine), especially of fluorine is desirable when a silicon nitride sintered body is used for mechanical parts at high temperatures.
However, it was found that although the reduction of content of halogen elements, especially fluorine, in silicon nitride sintered bodies is effective for improving high-temperature strength, the reduction of content of halogen elements, especially fluorine, tends to degrade room-temperature strength. Therefore, silicon nitride sintered bodies with a reduced halogen element, especially fluorine, content is not always preferable for applications in which room-temperature strength is more important, such as materials for sliding members and the like.
The present invention was devised considering such situations, and it is the object of the present invention to provide a silicon nitride sintered body which can preferably be used as a material composing mechanical parts and the like which are required high room-temperature strength.